Obstructive sleep apnea (OSA) is a highly prevalent disease with major neurocognitive and cardiovascular sequelae. Obesity is a major risk factor for OSA, but the underlying mechanisms remain unclear. Given the rising prevalence of obesity and the lack of adequate therapies for some afflicted patients, further mechanistic work is clearly required. Bariatric surgery is being done increasingly with compelling outcome data emerging; however, clinical response to weight loss is highly variable. In some patients, OSA is not present at baseline, despite morbid obesity, in other patients, OSA resolves following bariatric surgery, while other patients have persistence of OSA despite weight loss, and still other patients can experience re-emergence of OSA in long term follow-up studies after bariatric surgery. OSA can occur due to several major pathophysiological factors including pharyngeal anatomy, pharyngeal dilator muscle dysfunction, unstable ventilatory control, end- expiratory lung volume and arousal threshold. As a result considerable complexity exists in the obesity/OSA relationship, suggesting the need for further research. First, we will perform a baseline evaluation of pathophysiological traits among obese people prior to weight loss surgery. Because some people will have OSA and some will not, we will define the potential mechanisms underlying OSA and potential protective mechanisms among obese people without OSA (pharyngeal critical closing pressure Pcrit primary outcome). Second, we will re-evaluate these same individuals from the standpoint of sleep study and pathophysiological traits six months following bariatric surgery after a variable degree of weight loss. We anticipate that some OSA patients will have resolution of OSA whereas others will have persistence of disease. For non-OSA patients undergoing weight loss, we will have a positive control group which will allow us to account for non-specific effects of weight loss. This aim will allow us to test the hypothesis that pharyngeal mechanics is the predominant mechanism whereby weight loss leads to improvement in OSA. Third, we will perform magnetic resonance imaging during wakefulness at functional residual capacity, total lung capacity and residual volume on participants at baseline and 6months following bariatric surgery. This aim will allow us to perform structure/function assessments in our participants, to define the impact of weight loss on pharyngeal anatomy, and to quantify the lung volume dependence of the upper airway before and after weight loss. The acquired data will also be used for computational modeling including dynamic assessment of pharyngeal function during tidal breathing. As a result of the proposed research, we are confident that we will gain major insights into the as yet unanswered question ?why does obesity (sometimes) cause sleep apnea?. This research will have major therapeutic implications as it will allow us to individualize therapy for afflicted patients.